Newton s Laws of Motion Supplemental Text Material Pages 300-320
Sir Isaac Newton Born 1642 1665 began individual studies Proved universal gravitation Invented the Calculus Reflector telescope 1672
First Law of Motion Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. CONTINUES = INERTIA
Mass How much matter Measure of inertia
Weight Force upon object due to gravity Proportional to mass Pounds weight Kilogram mass ~2.2 lbs. (in Earth s gravity) Weight=mass x acceleration of gravity
Weight Weight=mass x acceleration of gravity Mass in kg, g is m/s 2 Newton force unit of weight
Mass vs. volume Mass How much matter Weight Force on the mass Volume How much space
IN CLASS ACTIVITY What is the weight of a two kilogram brick? Weight=mass x acceleration of gravity Mass=2 kg g=10m/s 2 Weight=(2 kg)(10m/s 2 )= 20 kg m s 2 = 20N
Second Law of Motion The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. Means acceleration ~ Force/mass
Weight is a Force Function of g (acceleration of gravity) Proper units: Different planet different weight for the same mass mg = F F g = m
Acceleration Change in velocity over time a = velocity/time a = F m Force over mass How can this be?
Acceleration Change in velocity over time a = velocity/time m/s s Definition of velocity
Acceleration Change in velocity over time a = velocity/time m/s = m/s 2 s Force over mass F a = m kg m/s 2 = m/s 2 How it is produced kg Definition of its components
Weight is a Force a = F m F = ma
Force is a vector Has direction Has magnitude Net force accelerated in direction of sum of impressed forces Added, subtracted, solved for diagonals
Force applied to puck
Accelerate the Puck Same direction, it goes faster Opposite direction, goes slower At angles, solve with trig or Pythagorean
Accelerate the Puck Push 6 Newtons cross-ice Push 8 Newtons up-ice Solve with Pythagorean Theorem c 2 =a 2 +b 2
Accelerate the Puck 8 2 + 6 2 = 64 + 36 = 100 c 2 = 100 c = 10 N net force on puck
Accelerate the Puck Push 12 Newtons up-ice Push 5 Newtons cross-ice What is net force?
Accelerate the Puck (5N) 2 + (12N) 2 = 25 N 2 + 144 N 2 = 169 N 2 c 2 = 169 N 2 c = 13 N net force
a = F m Double the force Also double the mass Direct proportion constant ratio
Freely falling objects Boulder vs. feather F a = a = F m m
Gravitational force interaction F = a m F = m a Fm a =
Zero Acceleration Motionless objects Downward force created by gravity Upward force created by surface
Zero Acceleration Push down on spring Spring pushes up on you Each molecule of table acts like microscopic spring pushing up on object
Zero Acceleration Cart across room Net force is zero Force applied pushing = frictional force
Friction Works against forces Opposite direction Not dependent on speed Not dependent on area of contact Only dependent on weight
Third Law of Motion Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Force is an interaction between objects Action reaction pairs
Action reaction pairs Forces of atoms within objects are in action reaction pairs No net acceleration due to these molecular forces External force needed to move object
Action reaction pairs Force on object moves it Force by object acts on other things Always equal
Action reaction pair Hammer exerts force on nail Nail exerts equal force on hammer
Action reaction pairs Consider recoil of firearm Force on bullet and on rifle the same Bullet has less mass than rifle Bullet has greater acceleration F m = F m = a
Action reaction pairs Rifle has less acceleration than the bullet
Action reaction pairs Rocket accelerates upward Recoil from exhaust gas
Action reaction pairs Birds push down on air Air pushes up on bird Fish pushes backward on water Water pushes forward on fish
Summary of laws of motion Newton s First Law of Motion Object at rest tends to remain at rest Objects in motion tend to remain moving Law of Inertia Function of mass of object Changes in motion occur due to presence of net force acting on object
Summary of laws of motion Newton s Second Law of Motion Acceleration proportional to net force a F m
Summary of laws of motion Newton s Third Law of Motion Objects exert equal and opposite forces upon one another Action reaction pairs have no net force